In the production of wood chips it is common practice to feed whole trees and parts thereof to rotary chip-forming machines usually referred to as chippers. During the timbering operation the felled trees are often dragged along the ground for a considerable distance, and as a consequence the branches and trunk may pick up particles of dirt in the form of soil and small stones. Most of this dirt mixes with the wood chips as the latter are formed in the chipper. The wood chips are also inherently mixed with particles of bark either in loose form or bound to the chips to varying degrees.
For some purposes, such as paper making, the proportion of dirt and bark must be low, say no more than 2%. Even with care in the handling of the felled trees this limit will often be exceeded. The problem will usually be more severe when the felled trees are converted to chips at the timbering site because there is less opportunity to remove dirt from the trees under these conditions.
The present invention provides methods and apparatus which aid in the production of wood chips having a reduced content of dirt and bark.
According to one important aspect of the invention there is provided for use at a timbering site a wheeled mobile trailer, which carries a power source such as an internal combustion engine and a rotary perforated cylinder type of separator driven by the power source, the cylinder being adapted to receive a stream of wood chips from a chipper. The chipper may be mounted on the chassis and driven by the power source or it may be a separate unit brought to the site independently of the chassis. During rotation of the cylinder the solids entering at the feed location slide and roll along the rotating perforated surface presented by the wall of the cylinder. Particles of dirt and bark pass through the perforations in the wall and are collected and transferred away, as by means of one or more conveyor belts running the length of the cylinder. Movement of the solids along the rotating perforated surface presented by the wall of the cylinder is obtained by inclining the cylinder, or if desired by providing one or more continuous helical blades on the inner surface of the wall to direct the solids either unidirectionally toward one end or in opposite directions from a central feed location toward both ends. In any case, wood chips having a reduced dirt and bark content, spill or pour by gravity from one or both ends of the cylinder and are transferred to a truck or other haulage vehicle. The means for transferring the chips is preferably a chip slinger or blower having an inlet receiving the chips from the cylinder and a rotary blade mechanism for slinging the chips in the form of a continuous stream whereby the chips can be raised to the height of the body of the haulage vehicle.
According to another important aspect of the present invention the dirt and bark content of initially produced wood chips is reduced by slinging or blowing the chips against an impact surface in a manner such that particles of dirt and bark are dislodged from the chips and in a manner such that the particles and chips are deflected into a classifier which then separates the particles from the chips. When the deflector or impacter is used in conjunction with a rotary perforated cylinder type of separator such as referred above the impact surface may be the surface of a rigid plate mounted inside the cylinder. In this embodiment the chips are slung or blown in the form of a continuous stream into the cylinder through one end thereof and after striking the plate they and the dislodged dirt and bark fall by gravity to the rotating surface presented by the wall of the cylinder.
Rotary perforated cylinder type separators are, of course, known but so far as the applicants are aware such separators have not been modified and used in the manner disclosed and claimed herein for lowering the dirt and bark content of wood chips. Examples of prior perforated cylinder type separators are described in U.S. Pat. Nos. 1,677,862 (separation of sand from ceramic pieces), 1,948,606 (sorting of comminuted ores and coal), 2,303,222 (separation of dirt from onions and sorting of onions by size), 2,769,543 (separation of fibrous particles from grannular cellulose acetate) and 3,087,499 (separation of peas from hulls).
Chippers for cutting logs and branches into chips of generally uniform size are described in U.S. Pat. Nos. 3,332,461 and 3,392,763.